AsianScientist (Feb. 7, 2013) – The peaks of the Himalayas are a modern remnant of massive tectonic forces that fused India with Asia tens of millions of years ago. Previous estimates have suggested this collision occurred about 50 million years ago, as India, moving northward at a rapid pace, crushed up against Eurasia.
Now researchers at MIT have found that the collision between India and Asia occurred only 40 million years ago – ten million years later than previously thought.
Oliver Jagoutz, assistant professor of geology in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says the results, which will be published in Earth and Planetary Science Letters, change the timeline for a well-known tectonic story.
“India came running full speed at Asia and boom, they collided,” says Jagoutz, an author of the paper. “But we actually don’t think it was one collision… this changes dramatically the way we think the India/Asia collision works.”
In particular, Jagoutz says, the group’s findings may change scientists’ ideas about the size of India before it collided with Asia. At the time of collision, part of the ancient Indian plate – known as “Greater India” – slid underneath the Eurasian plate.
“The real question is, ‘How great was Greater India?'” Jagoutz says. “If you know when India hit, you know the size of Greater India.”
By dating the Indian-Eurasian collision to ten million years later than previous estimates, Jagoutz and his colleagues conclude that Greater India must have been much smaller than scientists have thought.
“India moved more than ten centimeters a year,” Jagoutz says. “Ten million years [later] is 1,000 kilometers less in convergence. That is a real difference.”
To pinpoint exactly when the Indian-Eurasian collision occurred, the team first looked to a similar but more recent tectonic example. Over the last two million years, the Australian continental plate slowly collided with a string of islands known as the Sunda Arc. Geologists have studied the region as an example of an early-stage continental collision.
From their analysis of the literature, the researchers found that rocks high in neodymium and hafnium isotopes likely formed before Australia collided with the islands. Rocks low in neodymium and hafnium probably formed after the collision. Once the team identified the isotopic signatures for collision, it looked for similar signatures in rocks gathered from the Himalayas.
The team found a very clear signature: Rocks older than 50 million years contained exactly the same ratio of isotopes in both the northern and southern samples. However, Jagoutz found that rocks younger than 50 million years, along the southern boundary of the Kohistan-Ladakh Arc, suddenly exhibited a range of isotopic ratios, indicating a dramatic tectonic event. Along the arc’s northern boundary, the same sudden change in isotopes occurs, but only in rocks younger than 40 million years.
Taken together, the evidence supports a new timeline of collisional events: As India crept steadily northward, it first collided with a string of islands 50 million years ago, before plowing into the Eurasian continental plate ten million years later, sandwiching the string of islands, now known as the Kohistan-Ladakh Arc, between the massive continents.
“If you actually go back in the literature to the 1970s and ’80s, people thought this was the right way,” Jagoutz says. “Then somehow the literature went in another direction, and people largely forgot this possibility. Now this opens up a lot of new ideas.”
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Source: MIT; Photo: Pierre Bouilhol/MIT.
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